Cool summer due to lack of bullies El Nino, La Nina

Aug. 25, 2013
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Alex Martin takes cover from the rain while waiting for the start of a baseball game between the Washington Nationals and the Atlanta Braves on Aug. 17. Atlanta has seen an unusually cool, dreary summer. / David Goldman, AP

by Doyle Rice, USA TODAY

by Doyle Rice, USA TODAY

Why have much of the eastern and southern USA had such a cool summer? In part, it's because El Nino and La Nina, the big bullies of the climate playground, have been on vacation over the past few months.

El Nino (warmer-than-average water temperatures in the tropical Pacific Ocean) and its counterpart La Nina (cooler-than-average water temperatures) are unquestionably the world's climate drivers - and the main influences on weather in the USA.

The El Nino pattern is officially known as the "El Nino Southern Oscillation" (ENSO). And when either El Nino or La Nina are really strong, the world's other climate patterns run and hide.

That's not happened this summer: With tropical Pacific ocean temperatures neither unusually warm nor cold (known as an ENSO-neutral phase), the typically meek and little-known "East Pacific-North Pacific " (EP-NP) pattern has taken over, according to climate scientist Richard Heim of the National Climatic Data Center in Asheville, N.C.

This pattern is partly responsible for the weirdly cool and damp summer over parts of the eastern and southern USA. How cool? Seventeen states, mainly in the South, had a cooler-than-average July, according to the data center. And cities such as Washington, D.C., Atlanta, Nashville and Charlotte are all having an unusually cool August, the National Weather Service reports.

Also, Florida had its wettest July on record.

Unlike the tropical waters of El Nino, the "EP-NP" pattern deals with north Pacific sea-surface temperatures. This summer, water temperatures in the eastern north Pacific Ocean have been unusually warm, which Heim says has led to atypical weather patterns over most of North America.

"We've had a weird 'trough' of low pressure over the north-central USA," Heim says. (Low-pressure troughs typically bring cloudy and rainy skies.) And last weekend, a stationary front draped across the Gulf Coast was a typical winter location for the front, not one for summer, he says.

A 1995 study in the Bulletin of the American Meteorological Society found that unusually warm north Pacific ocean temperatures in the spring of 1993 could be linked to the disastrous flooding that inundated the Midwest that summer.

In addition to the EP-NP, other typically lesser-known climate patterns that come out of the shadows when there is no strong El Nino or La Nina include the North Atlantic Oscillation (NAO) and the Arctic Oscillation (AO).

Folks in the eastern USA may be getting more familiar with the NAO or AO, as they can sometimes strongly influence winter weather in the eastern USA., bringing heavy snow and bitter cold in some years.

Another reason that El Nino and La Nina get all the glory is because they can be forecast months in advance and can last for months, Heim says.

But with the big boys like El Nino staying out of the playground this summer, the EP-NP pattern is "getting its 15 minutes of fame," he says.